Sustainability & Environment (Agri-Food Systems) · Pollution Control & Environmental Engineering · also Air Pollution and Control, Air Quality Management, Atmospheric Pollution
Air pollutants, their health and crop effects, standards and indices, dispersion, and control equipment, plus noise. Agricultural sources (residue burning, ammonia, dust, odour) are covered here alongside the industrial ones GATE ES examines.
In the exams: GATE ES Section 6 (whole section, numerical); ICAR AICE subject 71 Unit 3; UGC NET Unit 6; state pollution control board recruitment
Air pollution is the presence in the atmosphere of substances at concentrations that harm people, crops, animals, ecosystems or materials, or that reduce visibility and amenity. It is the largest environmental risk to human health worldwide, and in South Asia it is among the most visible environmental problems, with severe seasonal episodes in the Indo-Gangetic Plains. Noise, unwanted sound, is a related physical pollutant that affects hearing, sleep and wellbeing. This subject covers pollutant sources and classification, standards and air quality indices, indoor air, effects on health, crops and materials, agricultural air pollution, the meteorology and mathematics of dispersion, control of particulate and gaseous emissions, vehicle emissions, emission inventories, and noise.
It builds on two companion subjects and does not repeat them. Environmental Chemistry explains atmospheric chemistry, ozone and smog formation, acid deposition and unit conversions; Atmospheric Science & Agrometeorology explains lapse rates, stability classes, inversions, plume shapes and wind measurement. Here those foundations are applied to air quality management. Monitoring instruments and quality assurance are treated in detail in the Environmental Monitoring & Instrumental Analysis subject.
GATE Environmental Science and Engineering Section 6 examines the subject in full, with numerical problems on Gaussian plume concentrations, settling velocity, collection efficiencies, emission estimates and noise levels. ICAR AICE Environmental Sciences (unit 3), UGC NET Environmental Sciences (unit 6) and state Pollution Control Board recruitment papers test pollutants, standards, effects, control devices and Indian rules. India's National Ambient Air Quality Standards, the National Air Quality Index and the Noise Pollution Rules are given as worked examples, alongside the WHO Air Quality Guidelines.
Air pollutants are classified in several ways. By origin, primary pollutants are emitted directly, such as sulphur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO), primary particulate matter, lead and many volatile organic compounds (VOCs); secondary pollutants form in the atmosphere from precursors, such as ozone, peroxyacetyl nitrate (PAN), sulphate and nitrate particles and secondary organic aerosol. By physical state, they are gases or particulate matter (aerosols: solid or liquid particles suspended in air). By chemical nature, they are inorganic or organic; and by source, natural (volcanoes, dust storms, wildfires, sea spray, pollen, biogenic VOCs) or anthropogenic. Particulate matter is classified by aerodynamic diameter. PM10 (particles up to 10 µm) are 'inhalable' and reach the upper airways; PM2.5 (fine particles up to 2.5 µm) penetrate deep into the lungs and are the most important for health; ultrafine particles (below 0.1 µm) can enter the bloodstream. Total suspended particulate (TSP) is an older measure of all suspended particles. Coarse particles come mainly from mechanical processes, such as dust and grinding; fine particles mainly from combustion and secondary formation. Other terms include dust (solid particles from mechanical processes), fumes (solid particles from condensed vapours, such as metal fumes), smoke (particles from incomplete combustion), mist (liquid droplets) and fly ash.
Sources are classified as stationary or mobile. Stationary point sources are individual stacks, such as power stations, cement kilns, refineries, smelters and boilers; area sources are many small sources treated together, such as households burning fuel, small industries, construction sites and agricultural fields; and line sources are roads, railways and flight paths. Mobile sources include vehicles, ships, aircraft and off-road machinery such as tractors and diesel generators. Major anthropogenic sources include fossil fuel combustion in power plants, industry and vehicles (SO2, NOx, CO, PM, CO2); industrial processes (cement and steel dust, metals, fluorides, VOCs, specific toxics); solvent use and fuel evaporation (VOCs); household combustion of solid fuels for cooking and heating (PM, CO, organic compounds); open burning of waste and crop residues; construction and road dust; and agriculture, the dominant source of ammonia. In Indian cities, source apportionment studies typically find large contributions from vehicles, road and construction dust, industry, household fuels and waste burning, with seasonal additions from crop residue burning and secondary particles formed from SO2, NOx and ammonia, though proportions differ widely between cities and seasons. Natural sources can dominate in some places and times: desert dust over north-west India and the Middle East, sea salt on coasts, wildfire smoke, volcanic SO2 and pollen. Transboundary and long-range transport means that much of the PM2.5 in a city may originate outside it, which is why airshed or regional management, rather than city-by-city control, is increasingly advocated.
Ambient air quality standards set maximum concentrations for pollutants in outdoor air, averaged over specified times, to protect health and welfare. In the United States, the Clean Air Act defines six 'criteria pollutants': particulate matter (PM10 and PM2.5), ozone, nitrogen dioxide, sulphur dioxide, carbon monoxide and lead. India's National Ambient Air Quality Standards (NAAQS), notified by the Central Pollution Control Board in 2009, cover 12 pollutants for industrial, residential, rural and other areas: SO2 (annual 50 and 24-hour 80 µg/m³), NO2 (40 and 80 µg/m³), PM10 (60 and 100 µg/m³), PM2.5 (40 and 60 µg/m³), ozone (8-hour 100 and 1-hour 180 µg/m³), lead (annual 0.5 and 24-hour 1.0 µg/m³), CO (8-hour 2 and 1-hour 4 mg/m³), ammonia (annual 100 and 24-hour 400 µg/m³), benzene (annual 5 µg/m³), benzo(a)pyrene (annual 1 ng/m³), arsenic (annual 6 ng/m³) and nickel (annual 20 ng/m³). Ecologically sensitive areas have stricter SO2 and NO2 limits. The World Health Organization's Air Quality Guidelines, revised in 2021, are not legally binding but set health-based levels much stricter than most national standards: for example, PM2.5 annual 5 µg/m³ and 24-hour 15 µg/m³; PM10 annual 15 and 24-hour 45 µg/m³; NO2 annual 10 and 24-hour 25 µg/m³; SO2 24-hour 40 µg/m³; and ozone 8-hour 100 µg/m³ with a peak-season guideline of 60 µg/m³. The WHO also gives interim targets to guide stepwise improvement. Emission standards, by contrast, limit what a source may release, expressed as concentration in flue gas (mg/Nm³), mass per unit of product or fuel, or grams per kilometre for vehicles. India sets industry-specific emission standards under the Environment (Protection) Rules and stricter limits for thermal power plants notified from 2015. Ambient standards and emission standards work together: emission limits are tightened until ambient air meets the standards.
An air quality index (AQI) translates concentrations of several pollutants into a single number and colour category for public communication. India's National Air Quality Index, launched in 2014, uses eight pollutants (PM10, PM2.5, NO2, SO2, CO, O3, NH3 and Pb) and six categories: good (0–50), satisfactory (51–100), moderately polluted (101–200), poor (201–300), very poor (301–400) and severe (401–500), each with associated health advice. For each pollutant a sub-index is calculated by linear interpolation between breakpoint concentrations: I = [(IHI − ILO)/(BHI − BLO)] × (C − BLO) + ILO, where C is the concentration and B and I the breakpoints of the band containing it. The overall AQI is the highest sub-index, provided at least three pollutants, including PM2.5 or PM10, are measured. For PM2.5 (24-hour), the breakpoints are 0–30 (good), 31–60, 61–90, 91–120, 121–250 and above 250 µg/m³ (severe). A PM2.5 concentration of 75 µg/m³ falls in the 61–90 band, so I = (200 − 101)/(90 − 61) × (75 − 61) + 101 ≈ 149, 'moderately polluted'. Visibility is reduced when particles and gases scatter and absorb light. Fine particles of about 0.1–1 µm, near the wavelengths of visible light, scatter most efficiently, so sulphate, nitrate and organic aerosols, especially when swollen by humidity, cause haze. The visual range is related to the extinction coefficient by the Koschmieder relationship, visual range ≈ 3.912/bext: an extinction coefficient of 0.2 per km gives a visual range of about 19.6 km. Visibility reduction disrupts road, rail and air transport, as in winter fog and smog episodes in northern India, and reduces sunlight reaching crops.
People spend most of their time indoors, so indoor air quality matters greatly for exposure. In low- and middle-income countries the dominant indoor problem is household air pollution from cooking and heating with solid fuels (wood, crop residues, dung, charcoal and coal) and kerosene in inefficient stoves, often in poorly ventilated kitchens. These produce very high concentrations of PM2.5, CO, polycyclic aromatic hydrocarbons and other pollutants, with women and young children most exposed. The World Health Organization estimates that household air pollution caused about 3.2 million deaths in 2020, from conditions including pneumonia in children, chronic obstructive pulmonary disease, lung cancer, ischaemic heart disease and stroke. In buildings everywhere, other indoor pollutants include environmental tobacco smoke; combustion products from gas stoves (NO2, CO); volatile organic compounds and formaldehyde from building materials, furniture, paints, adhesives and cleaning products; radon, a radioactive gas seeping from the ground, which is a leading cause of lung cancer in non-smokers; biological contaminants such as moulds, dust mites, bacteria and pet allergens, favoured by dampness; asbestos fibres from old building materials; and carbon dioxide from occupants, used as an indicator of ventilation adequacy. Sick building syndrome describes symptoms such as headache, eye irritation and fatigue associated with time spent in a building, often linked to poor ventilation and VOCs.
Indoor air quality is managed by source control, ventilation and air cleaning. Source control is most effective: replacing solid fuels with clean fuels such as liquefied petroleum gas (LPG), biogas or electricity; using improved, well-designed cookstoves with chimneys; banning smoking indoors; choosing low-emission materials; and controlling dampness. India's Pradhan Mantri Ujjwala Yojana (2016) expanded LPG access for poor households, though sustained exclusive use depends on the affordability of refills, and 'stacking' of old and new fuels is common. Ventilation dilutes indoor pollutants with outdoor air. It is expressed as air changes per hour (ACH), the volume of air supplied per hour divided by the room volume: a 200 m³ room supplied with 1000 m³ of outdoor air per hour has 5 ACH. Natural ventilation through windows and openings, local exhaust such as kitchen hoods and chimneys, and mechanical ventilation all help, though ventilation brings in outdoor pollution when outdoor air is poor. Air cleaners with HEPA filters remove particles; activated carbon removes some gases. For radon, sealing foundations and sub-slab depressurisation are used. At steady state, if a pollutant is emitted indoors at rate S into a well-mixed room of volume V ventilated at Q (m³/h) with outdoor air of concentration Co, the indoor concentration is approximately Ci = Co + S/Q, ignoring deposition and reactions, which shows why both reducing emissions and increasing ventilation lower concentrations. Exposure assessment combines concentrations in each microenvironment with the time spent there.
Air pollution affects the respiratory, cardiovascular, nervous and other systems. Fine particles (PM2.5) are the most harmful: they penetrate the alveoli, cause inflammation and oxidative stress, and increase deaths from heart disease, stroke, chronic lung disease, lung cancer and lower respiratory infections; there is no known safe threshold. WHO estimates that ambient air pollution caused about 4.2 million premature deaths worldwide in 2019, with South and East Asia most affected. Carbon monoxide binds haemoglobin roughly 200–250 times more strongly than oxygen, forming carboxyhaemoglobin that reduces oxygen delivery; high exposure causes headache, dizziness and death. Sulphur dioxide irritates the airways and causes bronchoconstriction, especially in asthmatics. Nitrogen dioxide irritates the lungs and increases susceptibility to infections and asthma. Ozone damages lung tissue and reduces lung function, especially during exercise. Lead is a neurotoxin that impairs children's cognitive development even at low levels; the phase-out of leaded petrol, completed worldwide by 2021, was a major public health success. Benzene causes leukaemia, polycyclic aromatic hydrocarbons such as benzo(a)pyrene are carcinogenic, and asbestos causes mesothelioma and asbestosis. Outdoor air pollution and particulate matter are classified by the International Agency for Research on Cancer as Group 1 carcinogens. Effects depend on concentration, duration and individual susceptibility: children, older people, pregnant women and people with heart or lung disease are most vulnerable. Short-term exposures raise hospital admissions and deaths on high-pollution days; long-term exposures reduce life expectancy. Episodes such as the 1952 London smog and the 1984 Bhopal gas disaster, in which methyl isocyanate released from a pesticide plant killed thousands, show the extremes of acute exposure.
Plants are affected by gaseous pollutants entering through stomata. Ozone causes stippling, flecking and premature senescence and reduces yields of sensitive crops such as wheat, rice, soybean and pulses, as the climate impacts subject explains; South Asia's high ozone levels are estimated to cause significant losses. Sulphur dioxide causes interveinal chlorosis and necrosis, and fluoride from aluminium smelters, brick kilns and phosphate fertiliser plants causes tip and margin burn and accumulates in forage, causing fluorosis in grazing animals. PAN damages young leaves; ethylene causes abnormal growth and flower drop. Particulate deposits reduce photosynthesis, and atmospheric brown clouds of aerosols reduce sunlight reaching crops. Acid deposition acidifies sensitive soils and lakes, leaching nutrients and mobilising aluminium, as seen historically in Scandinavia and north-eastern North America. Nitrogen deposition from NOx and ammonia over-fertilises natural ecosystems, reducing plant diversity. Air pollution also damages materials. Sulphur dioxide and acid deposition corrode metals and dissolve calcareous stone such as marble and limestone, forming gypsum crusts; concern over the yellowing and corrosion of the Taj Mahal led the Supreme Court to create the Taj Trapezium Zone with restrictions on polluting industries near Agra. Ozone cracks rubber and fades dyes; hydrogen sulphide blackens silver and lead-based paints; and particles soil buildings and clothing, raising cleaning costs. Visibility reduction and haze also affect tourism and transport.
In north-west India, farmers in Punjab and Haryana have widely burned rice straw after harvest in October and November to clear fields quickly for wheat sowing, because combine harvesters leave loose straw and the window between crops is short. Burning releases large amounts of PM2.5, black carbon, CO, NOx, VOCs and CO2, and coincides with low winds, temperature inversions and low mixing heights, so smoke contributes substantially to severe air pollution across the Indo-Gangetic Plains, including Delhi, though its share varies daily with meteorology and is debated. Burning also destroys soil organic matter and nutrients (nitrogen, sulphur and much of the residue carbon), kills soil organisms and causes road accidents through reduced visibility. Residue burning also occurs in rice and sugarcane areas elsewhere in Asia and in Africa and Latin America. Responses combine regulation, technology and incentives. In-situ management incorporates or retains straw using machines such as the Happy Seeder, which sows wheat directly through standing stubble, the super straw management system attached to combines, mulchers and zero-till drills; microbial decomposer solutions have been promoted to speed straw decomposition, with mixed field evidence. Ex-situ management collects straw for biomass power, compressed biogas, pellets, paper and animal feed. Shifting from long-duration to shorter-duration rice varieties widens the sowing window. India has provided subsidies for residue management machinery and custom hiring centres, banned burning under environmental law, and established the Commission for Air Quality Management in the National Capital Region and Adjoining Areas (2021) to coordinate action. Satellite fire counts are used to monitor burning.
Agriculture is the dominant source of atmospheric ammonia, mostly from livestock manure and urine (housing, storage and land spreading) and from nitrogen fertilisers, especially urea, whose hydrolysis produces ammonia that volatilises from alkaline and warm soils. Ammonia is not usually harmful at ambient levels by itself, but it reacts with sulphuric and nitric acids formed from SO2 and NOx to produce ammonium sulphate and ammonium nitrate particles, a large share of PM2.5 in agricultural regions, including northern India, Europe and China; it also deposits as reactive nitrogen, causing eutrophication and acidification of soils and waters. Reducing ammonia is therefore an effective and often cheap way to reduce PM2.5. Measures include incorporating or injecting manure and urea, using urease inhibitors and neem-coated urea, covering manure stores, low-protein feeding of livestock, and acidifying slurry. Agricultural dust comes from tillage, harvesting, unpaved farm roads, feedlots and wind erosion of bare soil; it contains soil particles, plant material, endotoxins and fungal spores, and can harm farm workers' lungs. Conservation tillage, cover crops, windbreaks and watering roads reduce it. Odour from livestock housing, manure storage and spreading, and from composting and food processing, is a frequent cause of complaints; it is caused by mixtures of compounds such as ammonia, hydrogen sulphide, volatile fatty acids, amines and mercaptans and is measured by dynamic olfactometry in odour units per cubic metre (the dilution at which half a trained panel can just detect it). Controls include covers, biofilters, scrubbers, good manure management and separation distances. Agriculture also emits pesticide spray drift and volatilised pesticides, smoke from burning pastures and forests, and the greenhouse gases methane and nitrous oxide, which are covered in the agricultural greenhouse gas mitigation and pesticides subjects.
Pollutant concentrations depend as much on the weather as on emissions. The atmospheric science subject explains lapse rates and stability; the key points for dispersion are these. When the environmental lapse rate exceeds the dry adiabatic lapse rate (about 9.8 °C per km), the atmosphere is unstable and vigorous vertical mixing dilutes pollutants; when it is less, especially under an inversion, the atmosphere is stable and vertical mixing is suppressed, trapping pollutants near the ground. Pasquill–Gifford stability classes run from A (very unstable, sunny days with light winds) through D (neutral, overcast or windy) to F (moderately stable, clear nights with light winds) and are used to select dispersion coefficients. The mixing height (mixing depth) is the height above the ground through which pollutants are mixed by turbulence. It grows during the day as the sun heats the ground, often reaching 1–2 km or more on sunny afternoons, and collapses at night to a shallow layer, sometimes only tens to a few hundred metres, under a radiation inversion. It is estimated from radiosonde temperature profiles, for example where the dry adiabat from the surface maximum temperature intersects the morning temperature profile (the maximum mixing depth). The ventilation coefficient, the product of mixing height and mean wind speed through the mixed layer (m²/s), indicates the atmosphere's capacity to disperse pollutants: a mixing height of 1500 m and a wind speed of 2 m/s give 3000 m²/s, and low values (often taken as below about 6000 m²/s) indicate high pollution potential. In winter in northern India, low mixing heights and calm winds combine with emissions to produce severe episodes.
Wind speed dilutes pollutants in proportion to the volume of air passing a source per unit time, and wind direction determines who is downwind. Wind speed increases with height in the surface layer, commonly described by the power law u = u1(z/z1)^p, where p depends on stability and surface roughness (about 0.07–0.15 in unstable to neutral conditions over open country and larger in stable conditions or over cities); for example, with u1 = 3 m/s at 10 m and p = 0.25, the wind at 100 m is about 5.3 m/s. A wind rose summarises the frequency of winds from each direction and speed class at a site and is used to site industries and monitoring stations so that prevailing winds carry pollution away from settlements, and to locate monitors downwind of sources. Plume behaviour reflects stability: looping in unstable air (high ground-level concentrations close to the stack intermittently), coning in neutral air, fanning in stable air (little vertical spread, but the plume may later fumigate), lofting when the plume is above an inversion (favourable), fumigation when a morning inversion breaks up from below and mixes a plume downward, and trapping when a plume is caught between a surface inversion and an inversion aloft. Topography modifies dispersion. Valleys trap pollutants under nocturnal inversions and drainage winds, as in the Meuse Valley (1930) and Donora (1948) disasters; coastal sites have sea breezes and fumigation; and urban heat islands create their own circulations. Tall stacks reduce ground-level concentrations near sources by releasing pollutants higher, but they spread pollutants farther, contributing to regional problems such as acid rain, which is why emission reduction, not dispersion alone, is the basis of modern control.
The Gaussian plume model estimates steady-state concentrations downwind of a continuous point source. It assumes a constant emission rate Q (g/s), a constant wind speed u (m/s) in the x-direction at the release height, no reaction or deposition, flat terrain, and that the plume spreads in the crosswind (y) and vertical (z) directions with Gaussian (normal) concentration profiles whose standard deviations σy and σz increase with downwind distance and depend on atmospheric stability. Reflection from the ground is represented by an image source below the ground. The concentration at (x, y, z) is C = [Q/(2π u σy σz)] × exp(−y²/2σy²) × {exp[−(z − H)²/2σz²] + exp[−(z + H)²/2σz²]}, where H is the effective stack height. At ground level (z = 0) on the plume centreline (y = 0), this simplifies to C = [Q/(π u σy σz)] × exp(−H²/2σz²). For a ground-level source (H = 0), the centreline ground-level concentration is C = Q/(π u σy σz). For example, with Q = 100 g/s, u = 5 m/s, σy = 100 m, σz = 50 m and H = 50 m, the ground-level centreline concentration is [100/(π × 5 × 100 × 50)] × exp(−2500/5000) = 1.27 × 10⁻³ × 0.607 ≈ 7.7 × 10⁻⁴ g/m³, or about 772 µg/m³. The same emission from ground level would give about 1273 µg/m³, showing the benefit of release height near the source.
The effective stack height H is the physical stack height hs plus the plume rise Δh, caused by the plume's momentum and buoyancy (hot gases rise). Plume rise is estimated with empirical formulas, such as the Holland formula and the Briggs equations, using exit velocity, stack diameter, gas temperature, ambient temperature, wind speed and stability; plume rise decreases as wind speed increases. Raising the effective height reduces maximum ground-level concentrations roughly in proportion to 1/H², and for a given stability, the maximum ground-level concentration occurs at the downwind distance where σz ≈ H/√2. India's guidelines have used empirical formulas linking minimum stack height to SO2 emission rate (such as H = 14Q^0.3, with Q in kg/h of SO2) and a minimum stack height for many industries. The dispersion coefficients σy and σz are read from Pasquill–Gifford curves or formulas (Briggs) as functions of distance and stability class; for the same distance, they are largest in unstable conditions (class A), giving high concentrations close to the source but rapid dilution, and smallest in stable conditions (class F), giving low near-source ground concentrations from elevated stacks but poor dilution farther downwind. Gaussian models are widely used for regulatory permitting and environmental impact assessment, including in India for assessing incremental ground-level concentrations of new projects. Regulatory models such as AERMOD (United States) improve on the basic equations with boundary-layer meteorology, terrain and building downwash. The models' limitations include the steady-state assumption, poor performance in calm conditions (concentration becomes infinite as u approaches zero), complex terrain, and the neglect of chemical reactions that form secondary pollutants such as ozone and sulphate, which require chemical transport models. Line sources (roads) and area sources use modified forms. Box models, which assume a pollutant is uniformly mixed in a volume defined by the mixing height and area, give simple estimates for urban airsheds: C = qL/(uH) for an area emission rate q, city length L, wind speed u and mixing height H.
Particulate control devices remove particles from gas streams using gravity, inertia, impaction, interception, diffusion and electrostatic forces. Their performance is expressed as collection efficiency η = (inlet − outlet)/inlet, overall or for particles of a given size (fractional or grade efficiency). Devices in series have overall efficiency η = 1 − (1 − η1)(1 − η2): a 90 per cent cyclone followed by a 99 per cent fabric filter removes 99.9 per cent overall. Particle settling in still air at low Reynolds numbers follows Stokes' law: terminal settling velocity vt = g ρp dp² ÷ (18 μ), where ρp is particle density, dp particle diameter and μ gas viscosity (about 1.85 × 10⁻⁵ Pa·s for air at 25 °C), assuming the gas density is negligible relative to the particle density. A 50 µm particle of density 2000 kg/m³ settles at about 0.15 m/s, whereas a 10 µm particle settles at about 0.006 m/s; because velocity varies with diameter squared, fine particles barely settle. Gravity settling chambers are large boxes in which the gas slows down and coarse particles settle; under laminar ('plug flow') assumptions, the efficiency for a particle size is η = vt L W/Q, where L and W are chamber length and width and Q the gas flow rate. They are cheap and simple but effective only for particles larger than about 50 µm, and are used as pre-cleaners. Cyclones make the gas spin so that centrifugal force throws particles to the wall, from which they fall into a hopper; they are effective for particles above about 5–10 µm, with efficiency rising with gas velocity and smaller diameter, and are characterised by the cut diameter d50, collected with 50 per cent efficiency (Lapple's method). Cyclones are widely used in agricultural processing such as cotton ginning, grain handling and rice mills, and as pre-cleaners.
Wet scrubbers capture particles by impaction and interception on liquid droplets or wetted surfaces. Spray towers are simple but less efficient; venturi scrubbers accelerate the gas through a throat where liquid is atomised, achieving high efficiency for fine particles at the cost of high pressure drop and energy use. Scrubbers can remove gases and particles together, handle sticky or explosive dusts and cool gases, but they produce wastewater and sludge that must be treated. Fabric filters (baghouses) pass dusty gas through woven or felted fabric bags; a dust cake builds up on the fabric and itself becomes the main filtering medium. They achieve very high efficiencies, often above 99.9 per cent, even for fine particles. The air-to-cloth ratio (gas flow per unit fabric area, m³/min per m², equivalent to filtering velocity) is a key design parameter: lower ratios give better performance and longer bag life. Bags are cleaned by shaking, reverse air or pulse jet. Fabric filters are limited by gas temperature (fabric materials have maximum temperatures), moisture and acid dew point, and by fire risk with combustible dusts. Electrostatic precipitators (ESPs) charge particles in a corona discharge and collect them on oppositely charged plates, from which they are removed by rapping. They handle large gas volumes at high temperature with low pressure drop and achieve efficiencies of 99 per cent or more, and are standard on coal-fired power plants. Efficiency is described by the Deutsch–Anderson equation: η = 1 − exp(−wA/Q), where w is the effective drift (migration) velocity of particles towards the plates, A the total collection plate area and Q the gas flow rate. With w = 0.1 m/s, A = 5000 m² and Q = 200 m³/s, η = 1 − e^(−2.5) ≈ 91.8 per cent; doubling the plate area to 10,000 m² raises efficiency to about 99.3 per cent. ESP performance depends on particle resistivity: very high-resistivity dusts, such as fly ash from low-sulphur, high-ash coal typical of India, are hard to collect and may need flue gas conditioning or larger ESPs, and very low resistivity dusts re-entrain.
Gaseous pollutants are controlled by absorption, adsorption, condensation, combustion and biological treatment, chosen according to the pollutant's chemistry, concentration and value. Absorption transfers a soluble gas from the gas stream into a liquid, in packed towers, plate (tray) towers or spray towers, usually with countercurrent flow of gas upward and liquid downward. Equilibrium follows Henry's law, as the chemistry subject explains, and absorption is enhanced when the gas reacts with the liquid, such as SO2 or HCl with alkaline solutions or ammonia with acidic solutions. Design uses mass transfer concepts such as the number and height of transfer units. Absorption suits soluble, acidic or basic gases. Adsorption retains gas molecules on the surface of a porous solid. Activated carbon, with internal surface areas of roughly 500–1500 m²/g, is used for VOCs, odours and mercury; other adsorbents include silica gel, activated alumina and zeolites (molecular sieves). Physical adsorption is reversible and favoured by low temperature and high concentration; chemisorption involves chemical bonding. Adsorption capacity is described by isotherms (Freundlich, Langmuir). In a fixed bed, the pollutant front moves through the bed until breakthrough, when outlet concentrations begin to rise; beds are then regenerated by steam or heat, recovering solvents in some cases, or replaced. Adsorption suits dilute VOC streams and solvent recovery.
Condensation removes vapours by cooling the gas stream or increasing pressure until the vapour condenses; it works for high concentrations of VOCs with relatively high boiling points and allows recovery, often as a pre-treatment before another device. Combustion (incineration) destroys organic gases and odours by oxidation to CO2 and water. Thermal oxidisers operate at about 700–1000 °C and depend on the 'three Ts': adequate temperature, residence time (typically 0.5–1 second) and turbulence for mixing. Catalytic oxidisers use a catalyst (often platinum-group metals) to achieve oxidation at lower temperatures (about 250–450 °C), saving fuel, but catalysts can be poisoned by sulphur, halogens and metals. Regenerative and recuperative designs recover heat to cut fuel use. Flares burn waste gases from refineries and oil fields in emergencies or where recovery is uneconomic. Combustion of chlorinated compounds can form acid gases and, if poorly controlled, dioxins, requiring downstream scrubbing. Biological methods use microorganisms to degrade biodegradable, water-soluble gases and odours: biofilters pass air through a moist bed of compost, wood chips or soil; biotrickling filters and bioscrubbers use packed or liquid systems. They are economical for large volumes of dilute odorous air, such as from composting, livestock buildings and wastewater plants, as the microbiology subject explains. Choosing a control technology depends on the pollutant (soluble, organic, valuable), concentration, flow, temperature, required efficiency, by-products (wastewater, spent carbon), energy use and cost. Pollution prevention at source, such as substituting low-VOC solvents, enclosing processes or changing fuels, is often more effective and cheaper than end-of-pipe control.
Sulphur dioxide comes mainly from sulphur in coal and oil. It is controlled by using low-sulphur fuels, coal washing to remove pyritic sulphur, fluidised bed combustion with limestone added to the bed, and flue gas desulphurisation (FGD). In wet limestone FGD, the most common method at large power plants, flue gas is scrubbed with a limestone slurry: SO2 reacts to form calcium sulphite, which is oxidised to gypsum (CaSO4·2H2O) that can be sold for wallboard and cement; removal efficiencies exceed 90 per cent. Spray-dryer (semi-dry) and dry sorbent injection systems use lime or other sorbents with lower capital cost. India notified stricter SO2, NOx, PM and mercury limits for thermal power plants in 2015, requiring FGD installation on many units over an extended timeline. Nitrogen oxides form during combustion in three ways: thermal NOx from the reaction of atmospheric nitrogen and oxygen at high temperatures (above about 1300 °C), fuel NOx from nitrogen in the fuel, and a small amount of prompt NOx. Combustion modifications reduce formation: low-NOx burners and staged combustion (creating fuel-rich and fuel-lean zones), flue gas recirculation and lower excess air all reduce peak temperatures and oxygen. Post-combustion controls reduce NOx to nitrogen: selective catalytic reduction (SCR) injects ammonia or urea over a catalyst at about 300–400 °C, achieving 80–90 per cent removal; selective non-catalytic reduction (SNCR) injects them at higher temperatures (about 850–1100 °C) without catalyst, with lower efficiency. Unreacted ammonia ('ammonia slip') must be minimised.
Carbon monoxide and unburned hydrocarbons result from incomplete combustion; they are reduced by good combustion (adequate air, temperature and mixing) and by oxidation catalysts. Volatile organic compounds from fuel storage and handling are controlled by vapour recovery systems at fuel depots and petrol pumps, floating-roof tanks and leak detection and repair programmes. Vehicles are a major source of NOx, CO, hydrocarbons and fine particles in cities. In petrol engines, the three-way catalytic converter simultaneously oxidises CO and hydrocarbons and reduces NOx, using platinum, palladium and rhodium; it works effectively only within a narrow band around the stoichiometric air–fuel ratio (about 14.7:1 by mass), maintained by an oxygen sensor and electronic fuel injection, and it requires unleaded fuel because lead poisons the catalyst. Diesel engines run lean, produce more NOx and particles, and need different controls: diesel oxidation catalysts, diesel particulate filters (which trap soot and are periodically regenerated by burning it off) and SCR systems using urea solution (diesel exhaust fluid) to reduce NOx, along with exhaust gas recirculation. Ultra-low-sulphur fuel (10 ppm) is required for these devices to work. India's Bharat Stage emission norms follow the European model; the country moved from Bharat Stage IV directly to Bharat Stage VI, nationwide from April 2020, with fuel sulphur limited to 10 ppm. Other vehicle measures include inspection and maintenance programmes, retirement of old vehicles, compressed natural gas for public transport (introduced in Delhi following Supreme Court orders around 2000–2002), electric vehicles, better public transport, and traffic management. Road dust resuspended by traffic and non-exhaust particles from brake and tyre wear remain significant even for electric vehicles. Agricultural tractors and generators are subject to separate, generally less strict, emission standards for non-road engines.
An emission inventory is a comprehensive, organised listing of the amounts of pollutants released by all sources in a defined area over a defined time. It underpins air quality management plans, dispersion and chemical transport modelling, source apportionment, regulatory compliance and tracking of progress. Inventories cover criteria pollutants, precursors (VOCs, NH3), toxics and greenhouse gases, the latter following IPCC methods covered in the carbon accounting subject. The basic estimation equation is E = A × EF × (1 − ER/100), where E is the emission, A the activity rate (fuel burned, product made, vehicle-kilometres travelled, hectares burned), EF the emission factor (mass of pollutant per unit of activity) and ER the overall emission reduction efficiency of control equipment in per cent. Emission factors come from compilations such as the US EPA's AP-42, the European EMEP/EEA Guidebook and national measurements; they are averages and may poorly represent individual sources. Mass balance is used where inputs and outputs are known: for sulphur dioxide from coal, nearly all sulphur in the fuel is emitted as SO2 unless retained in ash or removed by FGD, and because SO2 (64 g/mol) has twice the mass of sulphur (32 g/mol), SO2 emitted ≈ 2 × mass of sulphur burned × (1 − retention or removal fraction). A plant burning 1000 tonnes of coal a day with 0.5 per cent sulphur emits about 1000 × 0.005 × 2 = 10 tonnes of SO2 per day without control, and about 1 tonne with 90 per cent FGD efficiency. Direct measurement by continuous emission monitoring systems (CEMS) or stack sampling gives the most accurate data for large point sources.
Inventories distinguish point sources (individual stacks large enough to track separately, with location, stack height, temperature and flow, needed for dispersion modelling), line sources (roads, with emissions estimated from traffic counts, vehicle mix, speed and emission factors per vehicle-kilometre), and area sources (many small or diffuse sources, such as households, small industries, construction, fires, agricultural fields and livestock, estimated by allocating activity data across grid cells using proxies such as population or land use). Biogenic and natural sources, such as vegetation VOCs, soil NOx, dust and sea salt, are estimated with models. Bottom-up inventories sum source-level estimates; top-down approaches use observations, increasingly from satellites (such as nitrogen dioxide columns from TROPOMI) and inverse modelling, to constrain or check inventories. Uncertainties are often large for area sources such as residue burning, household fuel use, road dust and waste burning, which are especially important in South Asia. Spatial, temporal (hourly, seasonal) and chemical speciation (splitting VOCs and PM into components) allocation is needed for modelling. Source apportionment complements inventories by working backwards from ambient measurements: receptor models such as chemical mass balance and positive matrix factorisation use the chemical composition of collected particles (metals, ions, carbon fractions, organic markers such as levoglucosan for biomass burning) to estimate the contribution of each source type. Combining inventories, dispersion modelling and source apportionment is the basis of city clean air action plans, such as those prepared under India's National Clean Air Programme (launched in 2019) for cities not meeting the NAAQS.
Sound is a pressure wave in air; noise is unwanted sound. Because the ear responds to an enormous range of pressures, sound is measured on a logarithmic decibel scale. Sound pressure level Lp = 20 log10(p/p0), where p0 = 20 µPa is the threshold of hearing at 1 kHz; equivalently, in terms of intensity, L = 10 log10(I/I0). A tenfold increase in intensity adds 10 dB, and a doubling of intensity adds about 3 dB. Typical levels are about 30 dB in a quiet bedroom, 60 dB in normal conversation, 80–90 dB in heavy traffic or near a tractor, 110–120 dB at a rock concert and around 130–140 dB near a jet engine, at the threshold of pain. Decibels from separate sources are combined logarithmically: Ltotal = 10 log10 Σ 10^(Li/10). Two equal sources of 80 dB give 83 dB, not 160 dB; adding 85 dB and 82 dB gives about 86.8 dB. Sound from a point source spreading freely decreases by about 6 dB for each doubling of distance (inverse square law): a source producing 90 dB at 10 m gives about 78 dB at 40 m. For a line source such as a busy road, the decrease is about 3 dB per doubling of distance. Sound level meters apply frequency weightings to mimic human hearing; A-weighting, reported as dB(A), de-emphasises low frequencies and is used for most environmental and occupational measurements. Because noise fluctuates, statistical and time-averaged descriptors are used: the equivalent continuous level Leq (the steady level with the same energy over the period), L10 (exceeded 10 per cent of the time, representing peaks, used for traffic) and L90 (exceeded 90 per cent of the time, representing background), and the day–night level Ldn, which adds a 10 dB penalty to night-time levels.
Noise effects include noise-induced hearing loss from prolonged exposure to high levels, often first appearing as a dip in hearing sensitivity around 4 kHz, and tinnitus; sleep disturbance; annoyance and stress; interference with communication and learning; and cardiovascular effects such as raised blood pressure with long-term exposure to traffic noise, according to WHO's environmental noise guidelines. Occupational limits are commonly 85 or 90 dB(A) for an 8-hour day, depending on the jurisdiction, with an exchange rate of 3 dB (each 3 dB increase halves the permitted time) in most international practice or 5 dB in the US OSHA standard. Farm workers are exposed to high noise from tractors without cabs, threshers, grain dryers, chainsaws and livestock buildings such as pig houses. India's Noise Pollution (Regulation and Control) Rules, 2000, under the Environment (Protection) Act, set ambient limits in dB(A) Leq for day (6 am to 10 pm) and night (10 pm to 6 am): industrial areas 75 and 70, commercial areas 65 and 55, residential areas 55 and 45, and silence zones (at least 100 m around hospitals, educational institutions and courts) 50 and 40. The rules restrict loudspeakers and public address systems at night, with limited exceptions, and other rules regulate noise from generators, vehicles and fireworks. Noise is controlled at the source, along the path and at the receiver. Source control includes quieter machinery, maintenance, mufflers, vibration isolation, enclosures and lower speeds; path control includes distance, barriers (which are effective only when they block the line of sight and are close to the source or receiver), vegetation belts (which reduce noise only modestly unless very dense and wide), land-use planning that separates noisy and sensitive uses, and building insulation; receiver protection includes ear plugs and muffs, which typically reduce exposure by about 15–30 dB depending on fit, and limiting exposure time. Hearing conservation programmes at noisy workplaces combine measurement, engineering controls, hearing protection and audiometric testing.
A student uses this subject to master the GATE Section 6 numericals: Gaussian ground-level concentrations with and without stack height, wind power law, ventilation coefficient, Stokes settling velocity and settling chamber efficiency, ESP efficiency with the Deutsch equation, series efficiencies, SO2 emissions from coal sulphur, AQI sub-index interpolation, visibility from extinction, air changes per hour, and decibel addition and distance attenuation. Exams also test Indian standards, plume types, control device selection and the chemistry of FGD, SCR and catalytic converters.
A five-year practitioner, such as a pollution control board scientist, an environmental engineer in industry or an EIA consultant, uses the subject to inspect and permit industries, verify stack and ambient monitoring data, run Gaussian models for project impact assessment, select and size control equipment, prepare emission inventories and city clean air plans, investigate odour and noise complaints, and advise on residue management and ammonia reduction in farm areas.
A senior professional with 25 years' experience, such as a member secretary of a pollution control board, an air quality director of a city or a head of environment in a power or cement company, uses the subject to set strategy: prioritising sources by contribution and cost of control, coordinating airshed-wide measures across states, implementing power plant FGD and vehicle norms, designing incentives for residue management, and communicating AQI and health advisories during episodes.
A researcher with a PhD and 25 years' experience works on source apportionment, chemical transport modelling of PM2.5 and ozone, the health and crop impacts of air pollution, ammonia and secondary aerosol, satellite-based emission estimation, low-cost sensor networks and the evaluation of clean air policies. Such a researcher advises national programmes and trains modelling and measurement teams.
Standards and norms in this manual (India NAAQS 2009, AQI breakpoints, Noise Rules 2000, Bharat Stage VI, WHO 2021 guidelines) are given as published; India's thermal power plant emission limits and their compliance deadlines have been revised several times and are not quoted numerically. Source contributions to pollution in Indian cities, including the share of crop residue burning in Delhi's air, vary by study, season and day and are described qualitatively. Detailed design methods for cyclones, scrubbers and absorbers, chemical transport modelling, and occupational noise regulations of individual countries are outside the scope. Health burden estimates come from WHO and differ from other assessments such as the Global Burden of Disease study.
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